The ultimate alternative fuel source in many people's minds would simply be the air we breathe. There would be little to no harmful emissions, a cheap and renewable fuel source, and a relatively simple fuel delivery system. While the concept of a compressed air car has been around since the 1920s, recent improvements in air storage and compressed air engine technology have made a mass-produced compressed air car closer to realization. A modern compressed air car uses a special expansion valve to expand pressurized air inside a two-cylinder engine. The force of the expanded air drives down a piston, which in turn contributes to the turning of a crankshaft, much like a standard combustion engine.
Many inventors and motor companies have tried to create an efficient and powerful compressed air car over the years, but few have come close to putting their prototypes into mass production. Most recently, two companies called Motor Development International(MDI) and Tata Motors have announced plans to introduce compressed air cars into the marketplace, although few industry experts have actually have the opportunity to inspect their prototypes or verify many of the companies' claims. Because a compressed air car uses carbon fiber and other lightweight but strong materials for its body shell and storage tanks, many industry experts are concerned the vehicles would not survive stringent crash tests.
In order to save weight, a compressed air car's storage tanks are often made from carbon fibers, a technology also used on the US Space Shuttle. This is thought to be a positive development, since a carbon fiber storage tank would not create damaging shrapnel during an accidental or deliberate puncture. These storage tanks are routinely filled with compressed air to a pressure of 4500+ psi(pounds per square inch). Unlike electrical batteries, a compressed air tank would not lose much energy through leakage when not in use.
One major technological problem facing developers of a compressed air car is the nature of compressed air. As air becomes more pressurized, it generates a significant amount of heat. This heated air would affect the overall efficiency of the engine. Some manufacturers overcome this problem by installing heat exchangers similar to those found on turbocharged standard engines. These heat exchangers would interact with the heated air and draw off the excess heat through ventilation. If a compressed air engine required a powered cooling element, then it would not be nearly as energy-saving or emission-free as it should be.
Even if the heated air issue could be addressed with heat exchangers, there is an equal and opposite problem with compressed air as a fuel. Once the compressed air passes through an expander, it can become extremely cold. The engine's components and lubricants could freeze over, causing a loss of efficiency and potential damage from the cold. A workable heating system would have to be perfected in order to keep the expanded air from freezing. On the positive side, some of this extremely cold air could be used as air conditioning.
A compressed air car, at least at this stage of development, would not run completely on compressed air alone. Because the current models can only reach speeds of 40mph on compressed air alone, a standard combustion engine or electric motor would be necessary to reach standard highway speed. Proponents of the compressed air car point out that many drivers typically drive short distances in city traffic in order to accomplish most of their daily routines. The car would run almost completely on compressed air most of the time, with only an occasional need for assistance from a gas-powered engine.
Refueling a compressed air car could be as simple as connecting the vehicle to a standard air compressor for several hours, with the primary cost being electricity. Once the cars go into mass production, one plan calls for installing special high-powered air compressors at selected service stations. These air compressing units could fill a tank in approximately three minutes, and would also have heat exchangers to prevent the tanks from overheating during the refueling process.
Introduction of the current lines of compressed air cars would most likely start in smaller countries around the world, especially in places where gasoline is prohibitively expensive and pollution levels from fossil fuel burning are extremely high. Once the vehicles can pass the stringent standards of the US automotive industry and a sufficient number of fueling stations can be created, it is possible Americans may see a compressed air car on the road within a decade or so.